The present invention is directed to a photometer for sensing the optical density of liquids carried in a plurality of wells in a specimen tray in rows and columns, and more specifically, to an automatic device for indicating by row and column the particular well disposed in operative position relative to the photometer.
Conventional photometers are known for determining the optical density of a plurality of liquids contained in the individual wells of a microtray. The microtray is freely movable over a flat surface having an aperture therein. A light source or photocell is positioned above the aperture with the other being located in alignment therewith beneath the aperture so that the light will pass through the liquid in a well when the well is moved into alignment with the aperture. The rows of wells in the specimen tray are generally designated by letters while the columns are designated by numbers, so that a particular well would be identified by a letter and number combination. While some tactile device such as detent means are associated with the microtray and photometer to insure the exact positioning of each well in alignment with the optical system of the photometer, the particular wells undergoing examination can only be identified by careful visual scrutiny in tracing the row and column to the edge of the tray to determine the identifying letter and number. Such a procedure frequently leads to error in the identification of a particular well and the recording of the well identification and the value of the optical density for the particular well is a time consuming and tedious process which lends itself to the commission of additional errors.